Conservation and the intelligent use of energy resources are extremely important in today's society. In the case of electricity in particular, demands are constantly increasing and in many regions resources are strained. There have accordingly been many devices developed over the years to reduce electrical power consumption by electrical loads.
When used with inductive-dissipative loads such as ballasted gas discharge lamps or electric motors, the removal of a portion of each half cycle of the AC fundamental power can result in a significant decrease in the power factor. The power factor is the ratio of true power to apparent power, which can be expressed as:       power    ⁢                  ⁢    factor    =                    ∑        0        n            ⁢                          ⁢              [                              V            n            *                    ⁢                      I            n            *                    ⁢                      cos            ⁡                          (                              θ                n                            )                                      ]                            ∑        0        n            ⁢                          ⁢              [                              V            n            *                    ⁢                      I            n                          ]                            where            Vn=voltage of harmonic n    In=current of harmonic n    θn=phase angle between the voltage and current in harmonic n    n=harmonic number.
In purely resistive loads the voltage and current are always in phase, so power factor is not an issue. However, in inductive-dissipative and inductive-resistive loads (which includes circuits that behave like inductive loads such as ballasted fluorescent lighting and other gas discharge lamp systems), the line voltage and load current are almost invariably out of phase to some extent, so the power factor is typically less than one even where the supply power is uninterrupted. As the power factor decreases, the efficiency of inductive-dissipative and inductive-resistive loads also decreases. This is especially important to commercial and industrial electricity consumers because, apart from the negative effect of a poor power factor on power consumption, many utility companies charge a higher rate when the power factor falls below a specified level.
For example U.S. Pat. No. 5,455,491 issued Oct. 3, 1995 to Hajagos et al., which is incorporated herein by reference, describes an energy saving control circuit for use with ballasted fluorescent lights. The circuit comprises a power circuit connectable to an alternating circuit power supply, and a control circuit which includes means for timing the operation of a bi-directional switch in the power circuit, so that the switch supplies power to the load for a predetermined time during each half cycle of the power supply, and cuts off power to the load once during each half cycle. This circuit reduces power consumption, because the load does not consume power from the power supply during the interval of each half cycle in which power is cut off. However, in Hajagos et al. the switching circuitry interrupts the supply power only once during each half cycle of the supply power frequency. Accordingly, although the switching circuitry can have a fairly large tolerance for timing and can thus be relatively basic, the effectiveness of the control circuitry is limited.
U.S. Pat. No. 4,350,935 issued Sep. 21, 1982 to Spira et al., which is incorporated herein by reference, also describes an energy saving control circuit for use with discharge lamps. Spira et al. operates in a manner very similar to Hajagos et al., but Spira et al. interrupts the supply power multiple times during each half cycle of the power supply. This is a more effective control circuit than that described by Hajagos et al., but gives rise to switching problems. It is in general extremely important to ensure that an inappropriate element of the switch which circulates the current back to the load during interrupt intervals is never on for any substantial length of time while the power supply is being supplied to the load, because the surge current will destroy the circuit components. Accordingly, the switching must be very carefully timed taking into account not only the durations of the power supply and interrupt intervals, but also the latency of the switching devices. This is a practical problem that renders the control circuit of Spira et al. difficult to implement as described.
It would accordingly be advantageous to provide a control circuit for an inductive-dissipative or inductive-resistive loads that allows for interrupting the supply power multiple times during each half cycle of the power supply without risking destruction of the circuit components when switching due to timing overlaps or switching latency characteristics.